Private schools without religious affiliation spend almost twice as much per student as their public and Catholic counterparts and more than double that of other Christian schools nationwide, according to a new study.

In the Washington area, there are about 330 private schools with enrollments above 50 students, according to Education Department data. Two-thirds have some religious affiliation, and a quarter are members of non-Catholic Christian school associations. Although it is not surprising that some private schools spend more per student than public and faith-based schools, just how much more has not previously been documented.

"There are a lot of urban legends that drive the policy discussions," said Bruce D. Baker, a professor at Rutgers University and the author of the study. He said that private schools tend to be costlier than the commonly accepted figures in policy debates, especially conversations about school vouchers.

The secular private schools analyzed in the study spent $20,100 on each student in the 2007-08 school year vs. $10,100 in public schools. Nonparochial Catholic schools tended to spend roughly the same as public schools. (Parochial schools were not included in the study because their tax data are not publicly available and because their finances are so tied to those of the Catholic Church.) Members of two of the largest associations of Christian schools spent $7,100 -- several thousand dollars less per student than their public peers.

Per-student spending in the Washington region's public schools ranged from $10,400 to $19,300 that year, according to a Washington Area Boards of Education report, although public schools account for their spending differently from private schools, and numbers are difficult to compare directly.

The gap between what students pay in tuition at private schools and what their education actually costs can be wide. A Washington Post analysis of Education Department data and public tax records found that some of the area's top private schools spent thousands of dollars more per student than what they charged for tuition. At Maret School, for example, high school tuition was $26,820 for the 2007-08 school year. That year, the school spent $32,359 per student. At Potomac School in McLean, the gap was even larger, with tuition at $25,890 and spending at $35,665. Potomac School Head Geoffrey Jones said the gap was a conscious decision to keep the school affordable for a wider range of families.

The study's findings suggest that religious schools are the ones most likely to be accessible to students receiving school vouchers, which typically credit students far less than $10,000 per year for private schools.

The D.C. voucher program, for example, awards eligible students up to $7,500 toward the cost of private education. But average high school tuition last year at member schools of the Association of Independent Schools of Greater Washington was $24,700. Tuition at Archbishop Carroll High School in the District, a parochial school run by the Archdiocese of Washington where many students receive vouchers, is $9,250 this year. Average tuition at the 10 largest high schools in the D.C. area that are members of one of the Christian school associations examined in the study was $7,700 this year. And 82 percent of the D.C. voucher recipients in an Education Department study this year attended faith-based schools, most of them Catholic.

All of these schools offer financial aid, and students who receive vouchers to attend some the more expensive private schools in the Washington area often receive additional assistance from the schools themselves. But Baker, the study's author, said that the gap between tuition and actual expenses raises questions about how much voucher programs can expand.